Silica microspheres are drawn toward a catalytic micropump (center). Chemical reactions at the pump create an electric field, along with an electroosmotic force that pulls on the fluid surrounding the spheres.

Electrochemical reactions between hydrogen peroxide and a platinum-gold micropump (large gray disk) cause negatively charged silica spheres to first cluster at a distance from the pump (top). But as the spheres’ charge is neutralized by the chemical reactions, they begin to gather around the disk (middle) then arrange themselves into a crystalline structure (bottom). The disk is about 20 µm in diameter.

Credit: Langmuir

RULES OF ATTRACTION

Electrochemical reactions between hydrogen peroxide and a platinum-gold micropump (large gray disk) cause negatively charged silica spheres to first cluster at a distance from the pump (top). But as the spheres’ charge is neutralized by the chemical reactions, they begin to gather around the disk (middle) then arrange themselves into a crystalline structure (bottom). The disk is about 20 µm in diameter.

Credit: Langmuir

A microscale pump uses a simple chemical reaction to coax silica beads in a fluid to self-assemble into a crystalline structure (Langmuir 2014, DOI: 10.1021/la503118t). Such a device could provide a tiny motor for nanomachines, pull contaminants out of a fluid, or place coatings on nanoscale devices.

The catalytic pump consists of a gold anode and a platinum cathode. María José Esplandiu of the Autonomous University of Barcelona and her team placed 20- to 50-µm-diameter platinum disks on top of gold films sitting on a wafer. They subjected the pumps to one minute of oxygen plasma cleansing to remove any residual contamination and activate the surface then placed a gasket on top of the patterned pumps. They then mixed negatively charged 1.5-µm-wide silica spheres in a solution of hydrogen peroxide and added the mixture onto the pumps.

The hydrogen peroxide reacted with the metals, creating an electrical field oriented from the gold toward the platinum. This field triggers electroosmosis, causing the fluid containing silica spheres to flow toward the platinum disk. But because the spheres are negatively charged, the electrical field repels them, and they stop about 20 µm from the edge of the platinum. Gradually, however, protons produced by the chemical reactions bind to the spheres and neutralize their charge. The particles then begin to clump together, move toward the disk, and eventually build up into a crystalline structure surrounding the platinum.

Esplandiu says translating chemical energy into mechanical energy could be useful in nanofabrication, both by providing a power source and by guiding materials to self-assemble. She’d like to try replacing one of the metals in the pump with a semiconductor, which could absorb photons to create even stronger electrical fields.